To energize a superconducting magnet, electric current leads that connect current source equipment at room temperature with superconducting magnet at liquid helium temperature (4.2 K) for supplying a current of several ten amperes to several thousand amperes are used. The leads can also be used during magnet operation to assure a constant current flowing through the winding in the presence of non superconducting joints in the winding. These current leads are helium cooled to reduce the electrical resistance thereby suppressing not only the heat due to electric conduction but also the heat inflow by heat conduction through the cross section. The current lead conventionally used is a metallic copper wire with a large diameter which has low electrical resistance value. Although due to large diameter of the copper wire generation of Joule heat is reduced, however, heat penetration due to thermal conduction occurs to a side of the superconducting system at the same time. As a result, power loss of a crycooler and loss of the helium gas as a refrigerant due to the heat penetration become serious. When current leads are made using superconductor of zero electric resistance, the thermal losses due to resistance naturally becomes zero; the heat inflow is significantly reduced because of small cross section and the current capacity is several thousand times higher than the copper leads. Losses due to heat conduction can also be suppressed with the use of oxide superconductors which have low thermal conductivity. Due to these reasons, use of oxide superconductor which does not generate joule heat due to zero resistance and the small heat conduction due to their low thermal conductivity even if a large current is passed through it, is becoming high and at the same time these new leads save the electric power necessary for cooling to one third. Thus, current leads of oxide superconductors are very useful for both large scale as well as small scale applications. For example, in large scale applications such as in power generation and transmission lines high field magnets, current leads for low-Tcsuperconducting magnets, particle, accelerators, magnetic energy storage systems etc. Some of the small scale applications are in cryogenic experiments, scanners in space crafts, low current magnets, electronic devices (communications/data systems and sensors etc.) and magnetometers etc.
Although, the higher electrical and lower thermal conductivity properties of oxide superconductors make them the better choice; however, their poor mechanical properties particularly in sintered bulk form limits their frequent use. Various efforts have been made to achieve all these critical properties to their best values. In conventional methods, wires of ductile materials are made by extrusion or by rolling and drawing processes. These processes cannot be directly applied to the oxide superconductors, due to their brittle nature and incapability of plastic deformation. Several variations of these processes are being developed, however, to adapt them to ceramic materials. These include the powder-in-tube (PIT) process used to fabricate metal-clad wires, plastic extrusion process used to fabricate bare wires and rods, floating zone, melt casting/cold isopressing techniques for fabricating bare rods; and cold isopressing for fabricating tubes.
The powder-in-tube process uses an adequate calcined oxide superconducting powder packed in a nominal ductile metal tube [K. H. Sandhage, G. Riley, and W. L. Carter, J. Minerals metals and material society, vol. 43, p. 21(1991); P. Halder, J. G. Hoehn, Jr. U. Balachandran, and L. R. Motowidlo, J. Electron. Materials, vol. 22, p. 1294 (1993); K. Sato, T. Hikka, H. Mukai, M. Ueyama, N. Shibuta, T. Kato, T. Masula, M. Nagata, K. Iwata, and T. Mitsui, IEEE Trans. Magn. MAG27, p. 1231(1991); R. Flukiger, B. Hensel, A. Jeremie, A. Perin, and L. C. Grievel, Appl. Supercond. vol. 1, p. 709(1993)]. A wire is rolled, drawn or both rolled and drawn in successive stages until the required diameter generally of 1 mm achieved. This wire is would into a coil and subsequently sintered under suitable optimized conditions so that mechanical strength itself was upgraded and reporting Jc of the order of 104 to 105 A/cm2 at 77 K in self field.
Although this method results in an oriented grain structure, however, drawback of this method is that there is a problem of not only degradation of contamination of the metallic cladding but also of losses due to high thermal conductivity of the cladding material.
Bare wires and/or rods made from extrusion, melting, cold isopressing methods and tubes made from cold isopressing technique are useful for applications like current leads where metallic cladding is undesirable. For example, for most of the large applications, such oxide current leads mostly made from rod/tube shape are currently being used in various equipments like cryogenic free cryocoolers. However, as compared to tubes not much effort has been made on the oxide current leads in rod shape because of their low Jc values as compared to those of their tube counter parts. The reason for this behaviour is the dominance of the self-field due to current and which is inversely proportional to the diameter (ø) of the rod. That is by making rods of smaller diameter can minimize the problem of reduction in Jc due to self-field. Apart from the problem due to self-field, thick rods also face the problem due to the presence of secondary phases which limit Jc. It is to note that the method of preparation of thick rods having diameter (4.2 mm) exist, however, those of the thin rods of diameter below 4.2 mm and approaching towards wire diameter (<2 mm) which can be very useful for small scale applications as mentioned above are not only scarce but also make use of either organic binders or lathe machine to fabricate thin rods. However, these methods have their own merits and demerits discussed below.
As is well established that [H. Kumakura, K. Togano and H. Maeda J. Appl. Phys., 67(1990)] the major factors influencing the properties particularly Jc of BSCCO rods are the orientation, size of the grains and their distribution, the composition of the phases present and the mass density, these methods adopt appropriate mechanical working and heat treatments steps to achieve a highly oriented with uniform and smaller grain size polycrystalline microstructure.
The conventional plastic extrusion process consists of several stages mixing of preformed superconducting powder with a set of monomeric organic additives to form a paste, extrusion of the said paste into wires/rods of desired cross sections, solvent drying of the said extruded rods, burning of binder from the said dried rods followed by sintering. Although this method gives direct rods and also induces a certain amount of orientation in the wire/rods, particularly on the surface, however, the combination of organics particularly, monomeric additives which do not lead to a good adhesion of particles and the processing parameters chosen could not result into an end product with desired and consistent electrical/superconducting and mechanical properties due to the crack formation during solvent drying and voids formation during binder burn out.
An improved version of the above method is disclosed in U.S. Pat. No. 6,191,074, by Ravi-Chander et al. using polymeric additives instead of monomeric additives and additional processing steps. This process involves formation of an extrudable paste by mixing superconducting material with polymeric additives that bridge between ceramic particles to provide inter particles adhesion, extrusion of the said paste into wires/rods, drying of the extruded rods, burning of binder, cold isopressing of the said rods followed by sintering in reduced oxygen environment with one intermediate cold isopressing step. With the use of polymeric additives and inclusion of cold isopressing of the binder burn-out rod before and in between sintering steps helped in achieving desired and consistent electrical and mechanical properties of YBCO and BSCCO superconducting wires/rods without degradation. The reported average value of Jc was about 650 A/cm2. However, this method suffers from the formation of fine cracks in the rod during intermediate cold isopressing.
The above problem of unavoidable crack and void formation due to the use of organic additives and solvents in the plastic deformation process can be prevented by using melt processes like floating zone and melt casting etc. Michishita et al. in Bismuth-based high-temperature superconductors, Edited by H. Maeda, K. Togano, Marcel Dekker, Inc. 1996, p. 253 disclosed a floating zone melt method for the fabrication of Bi-2223 rods. Electrical contacts were made by ultrasonic soldering of indium. Then reporting maximum Ic of about 4.4 A (calculated value of Jc=880 A/cm2) and contact resistivity ˜10−5-10−6 Ω-cm2 at 77 K for 0.5×1×15 mm3 rectangular bars with ultrasonically soldered indium contacts cut from the rods of 4 mm diameter and length of about 95 mm. Drawback of this method is that it resulted in to higher contact resistivity values.
Another drawback inherent in, this method is the requirement of very slow rates (<1 mm/hr) for the formation of high-Tc Bi-2223 phase which makes it a low output process in terms of product quantity. Moreover, the formation of reasonably good amount of high-Tc phase formation in this melt process has not been reported to date.
E. Yakinci in J. Low Temperature Phys; vol. 105, no. 516, p 1535 (1996) disclosed a melt casting technique for the fabrication of Bi-2223 rods having diameter of 13 mm and 200 mm long and adopted a new method of sintering by applying four stages direct current to the rods. In this method, a mixture of the raw materials Bi2O3, PbO, Ga2O3, SrCO3, CaCO3 and CuO was melted in platinum crucible. This highly viscous molten material was poured into the cold graphite crucible and obtained highly dense glass rod. Three steps sintering of this glass rod was performed by applying a direct current to the glass and reporting high Jc of 9700 A/cm2 at 4.2 K for the best optimized sample. However, these values are for small rectangular pieces (cut from these sintered rods) whose area has been calculated from cross-sectional scanning micrographs. Therefore, these values can not be considered to be representative of the as prepared rod which is to be used in devices. Further, this method suffers from presence of non-superconducting Ca, Cu rich phases even in the sample prepared under optimized conditions due to incongruent melting of the desired Bi-2223 phase (Flukiger et.al., Bismuth-Based High-Temperature Superconductors, edited by H. Maeda and K. Togano, Marcel Dekker, Inc. 1996,p. 319).
The problems of the above methods due to incorporation of organic additives or due to melting can be minimized by using another method of cold isopressing. This method involving cold isopressing of the calcined powder into rod followed by sintering with inclusion of one or two intermediate cold isopressing step which is known for spreading the intermediate Ca, Cu rich phases perpendicular to the packing direction thereby increasing density of the specimen during subsequent sintering steps has been disclosed in the following references with some variants.
In reference Bismuth-based high-temperature superconductors, Edited by H. Maeda, K. Togano, Marcel Dekker, Inc. 1996, p. 277 Yamada et al. and in EP Patent No.: CN1224246, Tiancheng et al. disclosed a cold isopressing (CIP) process of making Bi-2223 rod current leads. In the former method calcined powder synthesized from solid state route is packed into a rubber mold and formed into rods by CIP at 400 MPa obtained from heating of a mixture of carbonates/oxides to obtain a (Bi+Pb): Sr:Ca:Cu: ratio of 2:2:2:3. The cold isostatically pressed rods were sintered for 100 hours at 810-840° in an atmosphere of low O2 partial press (Ar: O2=12:1), and then the CIP of these sintered is performed again. Finally, silver is plasma sprayed to form 20 mm long Ag layers on both ends of the bulk rod. Then reporting Ic=220A, Jc=570 A/cm2 and Rc˜0.1-0.2 μΩ under self-field at 77 K for rods of dimension: φ=7.0 mm, L=200 mm. Whereas, the latter method make use of co-precipitation route for the synthesis of calcined powder which is then cold isostatically into rods followed by steps of sintering, cold isopressing pressing and then sintering. Then reporting advantages of high compactness and high current load capacity.
Unlike in both the above methods, where only one cold isopressing step in between two sintering was performed, in U.S. Pat. No. 6,216,333, Kojima et al. disclosed a method with inclusion of two intermediate cold isopressing steps in between three sintering (each sintering step is of 50 hours). After completion of final sintering, these sintered rods (diameter=7 mm and length=150 mm) were grounded by means of a lathe machine into final thinner rods (diameter=4.2 mm and length=145 mm) followed by a process of making end silver contacts and another step of sintering to obtain rod current leads. Then reporting high Ic=710 A (calculated value of Jc=5,100 A/cm2) at 77K in zero field for 145 mm long rods having 4.2 mm diameter. However, like the above methods, this method not only suffers from introduction of fine compaction cracks due to intermediate CIP but also cracks developed during turning. This not only hampers reliability of the final product but also makes it mechanically weak.
Although, the above CIP process of preparation of Bi-2223 rods and by using intermediate CIP steps in between sintering steps discussed in the above methods cause the intermediate liquid phases rich in Ca, Cu to spread perpendicular the packing direction and this leads to alignment of the Bi-2223 platelets during subsequent heat treatment thereby increasing density of the specimen, however, this method still suffers from the presence of Ca, Cu rich secondary phases. In addition, undesirable of fine compaction cracks due to intermediate cold iso pressings are also introduced and total sintering is about 200 to 250 hours. This tendency of undesired residual phases and fine compaction cracks increases as the diameter of the rods increases. Thus, hampers reproducibility and cost. It is further to add that, in the above prior art methods where entire sintering is done in rod shape only and that too without intermediate crushing and mixing steps, generally thicker rods of diameter not less than 4.2 mm have been reported.
To minimize the traces of secondary phases, to avoid the use of : any organic binder, intermediate CIP steps and/or turning after completion of final sintering to fabricate thinner rods, another method of preparation of Bi-2223 rods is disclosed in reference Japan. J. App. Phys, vol. 44, No. 11, p. 7943 (2005) by Padam et al. This method differs from the prior art methods in three steps. First: starting powder contains silver. Second: sintering is done by cold isopressing of modified calcined powder first in tube shape (unlike generally rod shape in all the above methods). Third, crushing the initially sintered tube into powder (which has rarely been done after initial sintering in the above methods) to obtain a homogeneous powder for final sintering. This initially sintered tube powder is cold isostatically pressed (pressure of 400 MPa) into 120 mm long rods of 3 mm diameter followed by a process of making of grooves at both ends of these rods. Then a three layer silver metal contact is made on these grooves using a metal spray gun. Finally, this entire assembly was sintered in air. This process reduced the total sintering duration by about 40 hours from the conventional method. Then reporting final Bi-2223 rods with rarely found secondary phases and these rods have Jc of the order of 785 A/cm2 (Ic=55.5 A) and contact resistance of the order 0.45 μOhm at 77 K in self-field. At the same time these rods have reasonably good fracture strength of about 115 MPa. Although this method led to relatively good quality Bi-2223 rods and that too in 40 hours shorter sintering duration, however, drawback of this method is that the obtained Jc and fracture strength values are still not high.
From the hitherto know prior art, as detailed above, it is clear that there is a definite need to develop an improved process for the preparation of oxide superconducting lead in rod shape and oxide superconducting current leads made thereby.